1. Field of the Invention
The present invention relates to an audio signal amplifier circuit and a portable telephone set and a portable electronic device using the same amplifier circuit and, particularly, to an audio signal amplifier circuit of a portable telephone set or a portable electronic device, which can amplify an audio signal with high tone quality even when a capacitor externally mounted to an audio output amplifier of an IC of the portable telephone set or the portable electronic device is reduced in size to such as a chip capacitor.
2. Description of the Related Art
A conventional portable magnetic tape player such as a headphone stereo or a DAT or a conventional portable disk payer such as a MD or a CD is driven by one dry battery or two dry batteries at most. Therefore, a power source voltage of an audio amplifier circuit is 1.2 V or 2.4 V at most. Further, the audio signal amplifier circuit must be compact and thin and, recently, the audio signal amplifier circuit is requested to amplify enough an audio signal in a low frequency range with high tone quality.
On the other hand, a power source voltage of an audio signal amplifier circuit of a portable telephone set of such as a portable telephone system or a PHS (Personal Handy-phone System), in which tone quality as high as the level of the audio signal amplifier circuit of the above mentioned portable magnetic player or the portable disk player is not required, is about 3.3 V and an improvement of frequency characteristics of the audio signal amplifier circuit has been required in order to improve an output voice quality. In addition, the audio signal amplifier circuit of the portable telephone set must be more compact, thin and light in weight compared with a portable audio equipment. Further, it is requested recently to amplify not only voice but also music.
A problem of the audio signal amplifier circuit of the portable audio equipment having thickness to be reduced resides in a capacitance of an output capacitor externally mounted to an output circuit of the amplifier circuit formed as an IC. When a cut-off frequency of the audio signal amplifier circuit in a low frequency range is set to about 20 Hz, a capacitor having a capacitance in a range from 330 xcexcF to 2200 xcexcF is necessary when a load, that is, an impedance of a loud speaker, is in a range from 4 xcexa9 to 32 xcexa9.
This problem can be solved by the technique disclosed in U.S. Pat. No. 5,787,182 which is assigned to the assignee of the present invention and in which it is possible to reduce a capacitance of an output capacitor of an audio signal amplifier circuit by about 30% at an appropriate signal frequency.
On the other hand, in an audio signal amplifier circuit of a portable telephone set having a casing smaller and thinner than that of a portable audio equipment, the externally attached output capacitor has to be not a electrolytic capacitor but a chip capacitor. In such case, the capacitance of the externally attached capacitor becomes about 30 xcexcF at most.
When an audio amplifier circuit is realized by externally attaching a capacitor having such small capacitance by applying the content of U.S. Pat. No. 5,787,182, frequency characteristics of the audio amplifier circuit in a low frequency range is degraded, causing voice to be hardly heard. Further, the technique disclosed in U.S. Pat. No. 5,787,182 includes a filter having a capacitor provided on an output side of an amplifier circuit, for compensating for the low frequency range characteristics thereof. Therefore, in a portable telephone set using a high frequency clock, there is another problem that the amplifier circuit tends to oscillate due to noise generated thereby. In order to solve this problem, it is necessary to separately provide an oscillation preventing circuit. In such case, however, the number of circuits, which are to be integrated, becomes increased correspondingly and the number of parts, which are to be added externally, is also increased. Therefore, the addition of the oscillation preventing circuit is not preferable as an audio signal amplifier circuit of a portable telephone set.
Incidentally, as an audio signal amplifier circuit requiring high tone quality, there is a portable electronic equipment such as PDA (Personal Digital Assistance). However, PDA involves a similar problem to that mentioned above.
An object of the present invention is to provide an audio signal amplifier circuit capable of amplifying an audio signal with high tone quality even when a capacitor to be externally attached to an audio output amplifier of the audio output amplifier formed as an IC is reduced in capacitance to such a chip capacitor.
Another object of the present invention is to provide a portable telephone set capable of amplifying voice with high tone quality even when a capacitor to be externally attached to an audio output amplifier of an audio output amplifier formed as an IC is reduced in capacitance to such as a chip capacitor.
A further object of the present invention is to provide a portable electronic equipment capable of amplifying voice with high tone quality even when a capacitor to be externally attached to an audio output amplifier of the audio output amplifier formed as an IC is reduced in capacitance to such as a chip capacitor.
In order to achieve the above objects, the audio signal amplifier circuit, which is usable as a unit, in the portable telephone set or in the portable electronic equipment and which is formed as an IC, comprises a differential amplifier circuit provided in an input stage of the audio signal amplifier circuit, a single-ended power amplifier circuit provided in an output stage thereof, an external output terminal of the IC, which is connected to an output line of the single-ended power amplifier circuit, a first resistor connected between a certain terminal of the IC other than the external output terminal and a feedback input of the differential amplifier circuit, a first capacitor connected between the external output terminal and a loud speaker as a load, a second capacitor between the certain terminal of the IC and a wiring line between the external output terminal and the loud speaker, a filter circuit provided on a signal input side of the differential amplifier circuit and including a second resistor and a third capacitor for attenuating signal components having frequencies in a middle and high frequency ranges and voltage follower means provided between the input stage and the output stage of the audio signal amplifier circuit, wherein the first capacitor is a small capacitor having a capacitance value in the order of 30 xcexcF or smaller and an attenuation characteristics of a series circuit of the first capacitor and an impedance of the loud speaker in a low frequency range is compensated for by an amount of relative increase of amplification gain in the low frequency range, which is determined by a time constant of the second capacitor and the first resistor and a time constant of the third capacitor and the second resistor.
Since, in the construction of the audio signal amplifier circuit mentioned above, it is possible to use a small capacitor having a capacitance in the order of 30 xcexcF. or smaller as the first capacitor, that is, the output capacitor, a chip capacitor can be used as the output capacitor. It becomes possible to amplify voice with high tone quality by compensating for the attenuation characteristics of the load circuit in the low frequency range due to reduction of capacitance of the output capacitor thereof by the filter circuits provided on the input and output sides of the audio signal amplifier circuit. Further, with the above mentioned construction, the audio signal amplifier circuit tends to oscillate due to the provision of the filter circuits on the input and output sides. In order to prevent the audio signal amplifier circuit from oscillating, the voltage followers are provided between the input stage and the output stage thereof to drive the current amplification loop thereby. Thus, the phase relation is regulated by this construction to prevent oscillation of the audio signal amplifier circuit. Therefore, it becomes possible to set the input dynamic range of the audio signal amplifier circuit large compared with the past without providing any special oscillation preventing circuit.
As a result, it becomes possible to easily realize a portable telephone set or a portable electronic device, which is capable of amplifying an audio signal with high tone quality even when the size of the capacitor of the output amplifier, which is externally connected to the IC, is reduced to the size of a chip capacitor.